A stereo ranging apparatus is known for measuring the distance to the target (i.e., imaging target) using two images taken by a stereo camera. The stereo camera has a first form including first and second cameras having parallel optical axes.
A second form including the first and second cameras having non-parallel optical axes (i.e., in the directions apart from each other) is also proposed in order to more widen the entire photographing range of the stereo camera than the above first form (e.g., see Patent Literature 1).
A stereo ranging apparatus disclosed in Patent Literature 1 calculates disparity between two images using an image area in two images that corresponds to an area overlapped in visual fields of both cameras (i.e., overlapped area), and calculates the distance to the target based on this disparity value. A non-overlapped area is used for object detection based on general information on object movement such as an optical flow.
However, in the above second form, a case frequently occurs where image heights distance from the image center corresponding to the principal point of a lens) of the target images projected on the overlapped area greatly differ from each other. The image area is sharp when the image height of the image area is small, while the image area is blurred due to the effect of a spherical aberration of a lens when the image height is large. Thus, the above second form measures the distance using the two image areas included in images taken by respective different cameras and having different “blurs,” There is a problem that ranging based on the two images having different “blurs,” deteriorates the accuracy of ranging.
In this case, “blur” represents the degree at which the light reflected from a single point of an object or the like expands over the spatial area without being converged into a single point by a lens. Also in the ease of an image, “blur” represents the degree of the state at which an image is unclearly taken when a ray of light reflected from the single point expands larger than the size of an image sensor of a CCD or the like.
A process for calibrating relative positions of a plurality of lenses or relative positions of a plurality of cameras forming a stereo camera is generally known. The former is a distortion correction process for correcting lens distortion, and the latter is a geometry correction process for collimating optical axes of both cameras (see, Patent Literature 2). However, these processes do not correct the blur due to the spherical aberration.
Patent Literature 3 discloses a method of correcting blur using a mechanism for adjusting focus of a lens.